It is sometimes necessary to know not only the weight of an object but also the relative position of the center of gravity of the object on a particular surface. For example, it would be desirable to know the position of the center of gravity of a load in a truck to alert a driver if a shift of the mass in a truck during transport creates a risk of the truck tipping when traveling around corners. It would also be desirable to know the relative location of the center of gravity of an object on a conveyer system to ensure that the object is centered for processing or packaging or is at risk of falling off the conveyor without visual monitoring. It is also desirable to know the relative position or shift of position of a patient in a hospital bed. Patients are often unable to be cognizant of their location relative to the edge of a hospital bed. In these instances, it is desirable to know when the patient approaches the edge of a bed to alert a nearby caregiver that the patient is in danger of falling off the bed. Some prior patents have attempted to address this problem in different ways.
U.S. Pat. No. 5,276,432 to Travis (“Travis”) discloses that a patient's weight is determined by locating four force cells in a supporting relation at the corners of the patient's bed. First, the bed is tared by measuring the bed weight from the summed output of the four force cells. Then, the weight from each of the four load cells is measured with the patient in place and the tare value is subtracted to obtain a net patient weight. Then, a center of gravity is calculated from one particular load cell. From this point of reference, the position of the patient can be calculated. To estimate the position, all load cell measurements are needed. The data must be fed into a multi-purpose computer and processed. Thus, an instantaneous position indication is not possible with this system. Moreover, the patient's position can only be estimated when the patient is in the region defined by the four corners of the bed.
U.S. Pat. No. 7,253,366 to Bhai (“Bhai”) uses four load cells located at the corners of the bed. The signals from the load cells are summed and the bed tare is subtracted to indicate the patient's weight. From a series of prior calibration measurements, a table of data is stored. The measurements are compared to the lookup table and stored, and a variation of the patient's weight from the preset table is used to indicate a change in position. Again, a multi-purpose computer is required to perform a variety of calculations.